Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are instruments which typically use a sharp tip to characterize the surface of a sample down to nanoscale dimensions. The term nanoscale as used for purposes of this invention refers to dimensions smaller than one micrometer. SPMs monitor the interaction between the sample and the probe tip. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular site on the sample, and a corresponding map of the site can be generated. Because of their resolution and versatility, SPMs are important measurement devices in many diverse fields including semiconductor manufacturing, material science, nanotechnology, and biological research.
The probe of a typical SPM includes a very small cantilever fixed to a support (i.e., a handle) at its base and having a sharp probe tip extending from the opposite, free end. The probe tip is brought very near to or into contact with a surface of a sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector such as an optical lever system as described, for example, in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as strain gauges, capacitance sensors, etc. The probe is scanned over a surface using a high resolution three-axis scanner acting on the sample support, the probe, or a combination of both. The instrument is thus capable of measuring the topography or other surface properties or nanomechanical properties of the sample. Cantilever probes can be made from conductive material, enabling measurement of electrical properties.
SPMs may be configured to operate in a variety of modes, including modes for measuring, imaging, or otherwise inspecting a surface, and modes for measuring nanomechanical properties of a sample. In a contact mode operation, the microscope typically scans the tip across the surface of the sample while maintaining a constant probe-sample interaction force. In an oscillation mode of operation, sometimes referred to as tapping mode, the tip of the SPM is oscillated while interacting with the sample at or near a resonant frequency of the cantilever of the probe. The amplitude or phase angle of this oscillation is affected by the probe-sample interaction, and changes in the oscillation are sensed.
As the probe is scanned over the surface of the sample, a probe positioning control system monitors the interaction of the probe with the sample surface such as, for example, deflection of the cantilever (in the case of contact mode), or changes in the oscillation amplitude or phase angle (in the case of oscillating mode). The control system adjusts the probe's position (or average position in the case of oscillating mode) relative to the sample to maintain a constant probe-sample interaction. The position adjustment thus tracks the topography of the sample. In this way, the data associated with the position adjustment can be stored, and processed into data that characterizes the sample. This data can be used to construct an image of the inspected sample's surface, or to make certain measurements of selected surface features (such as, for example, a height of the feature).
The probe position adjustment is effected by a cantilever positioning actuator that is driven by a driving circuit. Various technologies for cantilever actuators are known, including piezoelectric and magnetic transducers. The driving circuit generates a probe positioning signal, and amplifies the probe positioning signal to produce a driving signal that is applied to the actuator. The driving signal continuously repositions the probe's separation distance from the sample to track an arbitrary topography of the sample's surface. Accordingly, the driving signal has a bandwidth from zero hertz to a frequency associated with the maximum operating bandwidth of the SPM, which corresponds to the maximum speed at which the probe can track the topography of the surface of the sample.
Some of the more recent developments in SPM technology have focused on high-speed scanning that can provide scanning speeds at a video rate such that the sample can be observed in near real-time. This presents a number of challenges. For one, the cantilever probe should have a high enough resonance frequency to enable scanning over an arbitrary topography at a required video rate. Ideally, the cantilever should be as “fast” as possible so that larger areas can be scanned at the desired rate. Resonance frequencies on the order of 1 MHz or more are desired. At the same time, the cantilever probe should provide maximum sensitivity in terms of deflection amount for a given interaction force with the sample. A more sensitive cantilever probe can be used to minimize the forces exerted on samples during measurement, thereby obtaining better characterization of a sample's true properties.
Unfortunately, there is a trade-off between speed and sensitivity in cantilever probes. A softer, i.e., lower-spring-constant cantilever probe that is more sensitive tends to have a lower resonance characteristic, thereby being less fast. Improvement in both properties can be obtained by scaling down the cantilever probe's dimensions. Accordingly, a cantilever probe that is shorter, narrower, and thinner, is desired.
Scaling down the cantilever probe dimensions presents its own host of problems. The generally preferred fabrication process is a batch process utilizing nanoelectromechanical systems (NEMS) techniques applied on a wafer scale rather than on an individual basis. These techniques include thin film deposition and photolithography operations enabling the mass-production of cantilever probes. As the smallest dimensions of the cantilever probe are reduced to the sub-micron scale, the conventional fabrication processes become exceedingly difficult to control uniformly. This problem lies in the use of chemical etching and controlling the amount of etching of the thin membrane that is to become the cantilever arm.
Individually-fabricated high-performance probes can be made using techniques such as Electron Beam-Induced Deposition (EBID) of the probe tip as a step separate from formation of the cantilever arm. However, these single-device techniques are not amenable to mass production. Probes produced in this manner can cost 1-2 orders of magnitude more than mass-produced probes. Therefore, a solution is needed to enable batch (wafer-scale) production of cantilever probes having similar or better characteristics than individually-produced probes. The batch process means producing multiple devices or AFM probes in parallel on each wafer and processing one or more wafers simultaneously. Typical materials used for cantilever probes include cantilever arms made from silicon or silicon nitride (SixNy) film. Probe tips can be made from a variety of materials, though silicon is generally preferably because the tips can be made very sharp using relatively simple etching techniques and a thermal oxidation process in which a layer of silicon dioxide, (SiO2) is grown on the tip structure.
In the fabrication of a traditional cantilever probe made from a silicon cantilever arm with a silicon probe tip, the starting point is usually a blank silicon wafer. Cantilever arms are produced by reducing the blank wafer thickness in a certain patterned area, typically with a density of 300 devices in a 4 inch wafer. Such a wafer usually has variation of the thickness in some areas, typically +/−1 micron for a 300 micron-thick wafer. As the etching proceeds to produce the cantilever arm of 1 micron thickness, for example, etching must remove 299 microns of material a 300-micron wafer. Due to the +/−1 micron uneven thickness of the wafer, the removal of 299 microns of material will produce 1 micron-thick cantilever arms in an area of 300-micron thickness, 2 micron-thick cantilever arms in the area of 301 micron thickness, and no cantilever arm at all in an area of 299-micron thickness. The cantilever arm formation yield from the various processing steps of forming the cantilever arms, defined by cantilever arms with a thickness within a desired range, is only around 30% if the +/−1 um unevenness of the wafer thickness is equally distributed throughout the wafer (in regions having sizes exceeding the size of the cantilever arms). In an industrial-scale production environment, such yield is unacceptable. It has been very difficult to form a thin layer of silicon less than 1000 nm thick to be used as cantilever arms from a larger bulk material without either encountering a low production yield or large variation in cantilever arm thickness from one batch to another even if the etch rate is controlled precisely. In produced batches with relaxed tolerances, the thickness variation produces an exponentially greater variation in spring constant because spring constant is proportional to the third power of cantilever arm thickness.
Making cantilever probes with arms thinner than 1 micron is far more difficult. One approach has been to utilize a chemically dissimilar material, such as a thin film of silicon nitride (Si3N4) with an even thickness by chemical vapor deposition (CVD) and pattern the deposited material into a cantilever arm. In these conventional cantilever probes made with silicon nitride cantilever arms and silicon tips, the nitride cantilever arm can be made thinner and with a uniform thickness than the cantilever arm of an all-silicon probe because etching of material to form the silicon tip and handle has virtually no effect on the nitride cantilever arm. In this case it is much easier to control cantilever thickness and size. However, such a process creates other challenges, namely, with using silicon to form a sharp tip by anisotropic etching because there is no silicon material above the CVD-applied Si3N4 to form the tip. Typically, a serial (non-batch) process such as electron beam deposition is used to form a tip on a silicon nitride cantilever arm.
Other approaches have been proposed for batch processing, though with mixed results and limitations in scaling down the dimensions. Moreover, in batch processing to form the silicon tip and to pattern the nitride cantilever arm to its desired width dimensions and shape, conventional processing has often required photolithography operations to be performed on structures supported by only the thin nitride membrane. This results in breakage of around 10% of the cantilever arms of a typical batch, with another 10% being lost to other causes. Even with this reduced yield, the thinnest practical cantilever arm that can be made economically using the conventional process is around 200 nm.
More recent advances have introduced a buried layer technology in which, prior to cantilever probe formation, multi-layer structures are formed at the wafer level. In one type of structure called silicon-on-insulator (SOI), used in the process disclosed in Qingkai Yu, Guoting Qin, Chinmay Darne, Chengzhi Cai, Wanda Wosik, and Shin-Shem Pei, Fabrication of Short and Thin Silicon Cantilevers for AFM with SOI Wafers, 126 Sensors and Actuators A: Physical, Issue 2 (2006) pp. 369-374, a layer of oxide is grown on top of a first silicon wafer; then, a second silicon wafer is bonded to the top of the oxide to form a single silicon-oxide-silicon wafer or slab, from which multiple cantilevers can be formed by batch processing. The buried oxide layer separates the device layer on which the silicon cantilever arm and probe tip are formed, from the handle layer in which a large portion of silicon at the base of the cantilever is kept. This allows the handle layer to be etched using a chemical that attacks silicon but not silicon dioxide, to release the cantilever without also etching into the device layer. However, the device layer must still be etched using a time-controlled process to control the cantilever arm thickness. Reduced etchant concentrations may be used to slow down the material removal process for greater control, but doing so reduces the processing throughput and increases the expense of cantilever probe fabrication.
In U.S. Pat. No. 7,913,544, a fabrication process is proposed in which a buried oxide layer is used in the fabrication of a cantilever probe with a nitride film cantilever arm and a silicon tip. In this approach, the probe tip is formed with a base pad. Thereafter, the nitride film is deposited so that part of it binds to the silicon tip's base pad. The resulting cantilever structure has the base pad and cantilever tip at the free end of the cantilever arm. This approach can address the problem of having to pattern the thin film nitride cantilever arm after it has been released from the backside; however the base pad thickness is difficult to control for the reasons discussed above. The size and performance of the cantilever are also limited in this process due to challenges with aligning the base pad and nitride cantilever arm, and having a cantilever structure in which the effective cantilever arm is composed of both, nitride film, and the part of the base pad leading up to the probe tip.
In U.S. Pat. No. 7,182,876, a buried nitride-oxide multi-layer structure is proposed as the starting point for batch cantilever probe fabrication, with the nitride layer intended to be formed into the cantilever arm in later processing steps. The use of a buried nitride layer allows formation of the probe tip directly over the cantilever arm; however, this approach requires protecting the probe tip when the handle layer, made from the same material, is etched. Typical protocol for protecting the probe tip involves depositing a protective coating of nitride film over the tip, which resists the chemical etching of the handle layer. However, the protective layer must then be removed, and this removal etch will also etch the nitride film used for the cantilever arm. Therefore, the same problem of controlling the thickness of the cantilever must be solved with this approach as well.
In general the SPM probe performance is gauged by the operating bandwidth, which is proportional to the resonance frequency f of the cantilever, and level of force control which is inversely proportional to the cantilever spring constant k. Maximizing f/k, or commonly expressed as f2/k, is regarded as optimizing of the probe performance. According to Sader et al., Calibration of Rectangular Atomic Force Microscope Cantilevers, Rev. Sci. Instrum. 70, 3967 (1999), incorporated by reference herein, f2/k˜1/bhL, where L is the length, b is the width and h is the thickness. Consequently reducing all the dimensions of the cantilever arm in proportion provides much improved performance. In the extreme case, as shown by Ando et al., A High-Speed Atomic Force Microscope for Studying Biological Macromolecules, PNAS 98, 12468 (2001) the cantilever size is reduced to 2 um×8 um×0.1 um for width, length and thickness respectively. However, scaling down the probe size presents major challenge to batch processing for the many reasons discussed above. In addition to the difficulty of forming a tip on a small cantilever of the size reported in Ando et al., placement of the tip at the small cantilever's free end is also very difficult because of the lithographic error. The SPM field has thus relied on expensive and serial processes to produce such small probes.
An additional demand in SPM applications is that the probes of a given model type are substantially uniform from one to the next, meaning the spring constant variation is preferably smaller than 30%, resonance frequency variation is smaller than 30%, and tip position and geometric variation (tip height and apex radius variation) is less than 40%. It has been impractical to produce such probes in commercial quantities.
In view of the above, a solution to the numerous challenges of miniaturizing silicon tip cantilevers made with either silicon or nitride cantilever arms, is needed.